Cellular materials have been available for decades, but new opportunities for cellular materials are emerging. Novel manufacturing approaches have beneficially affected performance and cost. Additionally, higher levels of basic understanding about mechanical, thermal and acoustic properties have been developed. These provide an integrated pathway between manufacturing and design.
Cellular materials have high stiffness and yield strength at low density relative to other competing materials and systems. That is, the cellular materials may be laminated between opposed sheets of another material. This creates an opportunity for ultra-light structures, with integrally bonded dense face sheets. In addition, cellular materials have large compressive strains achievable at nominally constant stress. Metallic foams provide for high-energy absorption capacity which is advantageous in crash and blast amelioration systems. These materials may be used effectively for either cooling or heat exchange structures. Cellular materials incorporated within a design to form sandwich skins can provide the desired mechanical properties in a cost-effective manner as compared to alternative structures.
One method of making metallic foams involves gas expansion in foam casting. Another method for making metallic foams is based on gas expansion in foam casting or powder metallurgy. According to this method, metal powder is mixed with a foaming agent, for example a gas. Gas pressure is derived by either a dispersed particulate such as H2 from TiH2, high pressure generated within an entrapped inert gas, or a gas injected into a liquid metal. This mixture can then be extruded or cast into the structural shape required. It is very difficult to control pore size or orientation using these known techniques.
The powder metallurgical Fraunhofer-process is another method used to create metallic foams. In this method, a foaming agent is added to a metal powder that is then mixed. This mixture can then be compacted or extruded into sheets or bars that can then be formed into the component shape using conventional molding techniques. Again, this process has little control of the pore size or orientation. Furthermore, this method can be cost-prohibitive if used to create geometrically complex parts due to the molds required.
Recently, processes have been developed to attempt to address the problem of lack of control of pore shape and orientation. However, this process, referred to as the GASAR process, involves the use of molten metals and the injection of gases, and is therefore, technically complex and expensive. Furthermore, the GASAR process allows the use of only one pore or cell orientation in a component and the shapes of the components are generally limited to plates, rods, and tubes.
Methods of making ceramic foams and cermet foams, which contain both metallic and ceramic materials, encounter similar difficulties. Such methods are extremely complex and do not produce products with predictable quality or predictable porosity. Further, prior cermet foams utilize a foaming agent, and thus both the shape and the amount of porosity can not be controlled. As a result, it is difficult to make foams that have consistent mechanical properties. Additionally, the manufacture of complex shaped foams using those methods is not feasible.
While others have developed processes for the fabrication of metallic foam structures with oriented porosity, none of those processes are capable of creating a combination of open and closed cell porosity, nor are they capable of creating components directly from CAD designs. Additionally, control of pore size and pore orientation is difficult. Moreover, conventional processes do not provide for the fabrication of integral structures with a metallic foam skin in a cost-effective manner. Consequently, there is a need for a fabrication process that can produce complex metallic foam components with optimized dynamic mechanical properties in a cost-effective manner.
Further, there is a need to fabricate metallic foam structures that have both open and closed cell porosity. Open porosity is characterized by the amount of surface area that is accessible by a gas or liquid if the structure were to be immersed in it, while closed porosity is the porosity in the structure not accessible to a gas or liquid. The size, distribution, and aspect ratio of close-celled porosity in a foamed material can have a direct effect on its energy absorbing and blast amelioration capability and other mechanical properties such as compressive strength.
In addition, there is a need to fabricate foam structures using ceramic materials or combinations of metallic and ceramic materials. There is a need for a fabrication process that can produce complex ceramic and cermet foam components with optimized dynamic mechanical properties in a cost-effective manner.